NITROGEN VERSUS FLUORINE PROTONATION OF NF3 IN THE GAS-PHASE - A COMBINED MASS-SPECTROMETRIC AND GAUSSIAN-1 ABINITIO MO STUDY REVEALS THE EXISTENCE OF 2 DISTINCT ISOMERS F3NH+ AND F2N-FH+

The potential energy surface of [N,F3,H]+ ions, generated by the gas-phase protonation of NF3 using CH5+ and H-3(+), is probed by means of high-level GAUSSIAN-1 ab initio MO studies and mass-spectrometric techniques. The global minimum corresponds to the fluorine-protonated isomer F2N-FH+ (1). This ion/dipole complex is found to be 6.4 kcal mol-1 more stable than its nitrogen-protonated form F3NH+ (2). The barrier for the reaction 1 --> 2 is significant (52.6 kcal mol-1), thus preventing facile isomerization. Further, while the isomer F2N-FH+ has a low energy dissociation channel to produce NF2+ and HF, with a heat of reaction of < 14.6 kcal mol-1, the less stable isomer F3NH+ (2) is trapped in a deep potential well, which prevents both rapid isomerization and dissociation. The Potential energy surface explains the distinctly different kinetic energy releases (KER's) associated with the reaction: [N,F3,H]+ --> NF2+ + HF. The loss of HF from the ion/dipole complex F2N-FH+ gives rise to a small KER (T<0.5> = 21 meV), while the same reaction of F3NH+ (2) is associated with a dished-top peak and a large KER (T[0.5] = 740 meV).